I’m not sure if wagering for the Belmont was taking place already at the track itself, if so those wagers placed at the track itself would be refunded. There are some places that take advance wagering, or more specifically called “Futures Wagering”. It’s basically far in advance wagering, for those who wish to place a wager when they can get good odds since that futures bet “locks” the odds at that time for you. Those futures wagers are now lost for the bettor, those are not refunded.